The present invention relates to a deformable curvature mirror that is capable of controlled deformation by the application of electrical voltages to the mirror and, in particular, is directed to such a mirror that is for use in an adaptive optics system having a wavefront sensor for controlling the deformation of the curvature mirror.
There are various adaptive optics methods and devices which include a wavefront sensor for sensing the aberrations in the wavefront of light waves and then correcting or compensating for those aberrations, such as the atmospheric aberrations that effect the viewing of stars and planets through a telescope. The existing methods and devices for sensing and measuring the wavefront include several interferometric techniques, the Shack-Hartmann wavefront sensing techniques and various other systems involving the projection of patterns of light through an optical system. Such prior techniques and systems are typically complex and expensive, as well as having various inherent deficiencies. In addition to the deficiencies of the prior art wavefront sensors, the deformable mirrors that are controlled by those wavefront sensors for adaptive optics also include numerous deficiencies. For example, a stack actuator mirror is comprised of a multiplicity of push rods engaging the back of a flexible mirror and the extension-retraction of each push rod is usually controlled by a Shack-Hartmann wavefront sensor. The Shack-Hartman wavefront sensor measures local slopes of the wavefront and these slopes are fitted with a wavefront reconstructor which in turn generates a continuous surface, matching all the slopes. This type fitting is blind to hysterisis effects in the actuators thereby causing a waffle pattern to appear on the mirror surface. The push rods tend to produce a deformation which is nearly a straight line on the mirror surface between each pair of adjacent push rods that results in large fitting errors when a small number of actuators are used. Moreover, the number of push rods and, therefore, the closeness of the push rods is physically limited, as well as the length of their travel, whereby the accuracy and degree of optical correction that can be applied by the stack actuator type mirror is limited. Since all actuators have the same travel and are attached to a rigid reference surface, the mirror has the same stroke for all modes, i.e. low order focus has the same stroke as the highest mode produced by every other actuator being turned on and off. For correcting the aberrations originating in the atmosphere, this range of stroke at the highest modes is not necessary, whereby the corrections may not be accurate for small errors.
Further, some other wavefront sensing and deformable mirror techniques and devices are not directly applicable to all types of adaptive optics for correcting the wavefront to thereby correct the image.
In summary, it is an object of the present invention to provide a novel deformable curvature mirror that is capable of controlled deformation by selectively applying electrical voltages to produce a wide range of accurate curvatures for correcting the wavefront of light in an adaptive optics system. A further object of the present invention is to provide a deformable curvature mirror that includes a pattern of electrode segments on the backside of the mirror that are of a size, shape and arrangement for producing the mirror deformation necessary for the particular application of the mirror by selectively applying variable voltages to the individual electrode segments. A still further object of this invention is to provide a unique construction of a deformable curvature mirror that includes two plates of electro-restrictive or magnito-restrictive material adhered together in a bimorph and monomorph configuration. Typically two plates of similar PZT type are glued together with an adhesive, then a mirror surface is placed on one side, and a pattern of electrode segments is placed either between or on the back side, whereby the application of a variable voltage to the individual electrodes causes a charge to accumulate on the conductive layer thereby producing an electric field for causing a controlled expansion or contraction of the electro-restrictive plates in the individual areas defined by the electrode segments. In the case where the electrode pattern is on the rear plate, the front plate acts as a restraining member and with the expansion or contraction of the rear plate a local bending moment occurs for thereby producing a curvature of that section of the mirror. Still another object of the present invention is to provide such a deformable curvature mirror with one set of electrode segments for controlling the slope of the wavefront at the edges of the pupil and another set of electrode segments for controlling the curvature of the wavefront inside the pupil.